Sunday, September 18, 2011

The Warming of the Atlantic Ocean

There's an interesting paper by Lee et al in Geophysical Research Letters: "What caused the significant increase in Atlantic Ocean heat content since the mid-20th century?"

The warming is large: In the last 40 years the upper 700 meters of the Atlantic Ocean have gained heat at a rate of 20 ZJ/decade. (Here "Z" = "zetta" = 1021, which I used only because I've never had the chance to use it before.)

The Agulhas Current around south Africa

Other oceans are gaining heat too: the Pacific Ocean at 15 ZJ/decade, and the Indian Ocean at 5 ZJ/decade.

These are large amounts. For the Atlantic Ocean it works out to 60 TW (terawatts; tera = 1012), which is 4 times the amount of energy used by humans worldwide.

Why have these oceans warmed? You get one guess (hint: it's a three-letter word, and rhymes with "can."). Click here for the answer.

But why has the Atlantic warmed more than the Pacific or the Indian, when it comprises only 20% of oceanic surface area? That's more complicated, and I can't do it justice here, but mostly the heat it is coming from the Indian Ocean, much of it via the Agulhas Current flowing around south Africa, "which is augmented by the strengthening of the wind stress curl over the South Atlantic and Indian subtropical gyre." The amount of water "leaking" from the Indian Ocean to the Atlantic Ocean via this route is about 15 times that of all freshwater flowing into the world's oceans.

Aha, so Dickens wants a mention in the AR5. :) Interesting anyway, but re degrees of pleasantness don't forget that for the clathrate gun to be triggered under present circumstances would mean that the East Siberian Shelf methane would all go first. Not a happy thought.

Re the Atlantic, it's been thought for a long time that volcanism there (the Greenland magmatic province that got the North Atlantic split going in a big way) had a big role in the PETM. Details are unclear, and I'm not sure everyone in the field even agrees. There's some detail relating to this in the Dickens paper, but I haven't had time to more than skim them so far.

All of that said, the Agulhas warming isn't something that could do the job; among other things it's not deep enough. OTOH it's definitely helping warm the Arctic Ocean, regarding which see my remark above.

David, remember a few weeks ago when I told you that Ryan Maue was telling a porky when he claimed that the warming of the North Atlantic was due to the AMO? This paper says why.

Re the description, it's pretty straightforward:

The warm Agulhas heads down the east coast of Africa and in cool-climate conditions is largely forced off to the east by the very strong Antarctic Circumpolar Current as soon as it passes the tip. As the climate warms, the tropics expand and push the entire global atmospheric circulation poleward, including the winds that drive the ACC. This in turn enables more (and yet more as the warming proceeds) of the warm Agulhas water to continue around the tip into the Atlantic, where it gets caught up in the thermohaline circulation and carried north.

And the expansion of the tropics? It's been formally attributed to you know who.

William, I imagine it's the case that the Agulhas would have such an effect (although better attributed to the action of the ACC AIUI), and since is likely continuing to do so, but per recent obs it is indeed leaking more and more into the Atlantic. From the Lee et al. abstract: "Further analysis reveals that the increased inter-ocean heat transport is not only caused by the increased upper ocean temperature of the inflow but also, and more strongly, by the increased Agulhas Current leakage, which is augmented by the strengthening of the wind stress curl over the South Atlantic and Indian subtropical gyre." (Emphasis added.) In the last few years this has become a big focus of oceanography research.

Belette, are you sure about the direction of the Agulhas Current? What I'm reading, like the Wikipedia article and others, says the flow is from the Indian to the Atlantic, but that 85% of this is returned by a retroreflection. The remaining 15% is called the "Agulhas Leakage," and still constitutes 15 Sv.